In the prior art, various types of radars are used for the purpose of meteorological observations, air traffic control, and the like. Such radars detect the size, shape, distance, velocity, and the like of a target by emitting high-frequency radio waves, such as microwave, from an antenna towards the target, and receiving reflected waves or scattered wave from the target. For example, a meteorological radar for observing meteorological conditions emits radio waves toward water droplets, such as rain, and analyzes the reflected waves that are received to determine, for example, the size of the area of precipitation or the amount of precipitation.
Such radar normally implements a monostatic technique, in which signals are transmitted and received with one antenna and the connection of the antenna with a transmitter and a receiver is switched, and a bistatic technique, which a transmission antenna connected to a transmitter and a reception antenna connected to a receiver are used.
For the monostatic technique, there has been disclosure of, for example, a meteorological observation radar including a transmitter for generating and outputting pulse-shaped high-frequency signals, an antenna for emitting the high-frequency signals generated by the transmitter toward the atmosphere as high-frequency radio waves and receiving high-frequency radio waves reflected or scattered by a target, a receiver for receiving the high-frequency radio waves reflected by an object through the antenna, and a circulator serving as a switching means for switching between transmission of the high-frequency signals from the transmitter to the antenna and transmission of the high-frequency signals from the antenna to the receiver (for example, refer to patent document 1).
In recent years, the technology related to meteorological prediction simulation (numerical forecast model) has gone through a drastic improvement in accuracy and calculation speed due to the increased computer calculation processing speeds and development of various algorithms. Nowadays, there is a demand for higher density, higher time resolution, and higher spatial resolution of the initial value data used for the meteorological prediction simulation. The time resolution herein refers to the time necessary to collect a piece of observation data. The time resolution improves as the time becomes shorter. Such a case is referred to as a satisfactory time resolution. The spatial resolution refers to the size of the region including a target (reflecting body or scattering body) that is observable by the radar. The spatial resolution improves as the region becomes smaller. Such a case is referred to as a satisfactory spatial resolution.
In the radar described in patent document 1, however, the entire atmosphere above the ground surface undergoes beam scanning (hereinafter referred to as “volume scanning”). Thus, when observing clouds and rain droplets, the range resolution (resolution in range direction, namely, the direction in which the radar emits radio waves or the direction in which radio waves travel, the radial direction γ when the spatial coordinate system is defined with polar coordinates (γ, θ, φ)) is several tens of meters and relatively long. Further, the time resolution is also several minutes and relatively long. Accordingly, the resolution is insufficient from the standpoint of higher accuracy for a meteorological prediction simulation.
In other words, for a rapidly-growing target such as thundercloud, a time resolution of several minutes is too long to analyze the generation mechanism of the thundercloud. In addition, when the range resolution is several tens of meters, fine dispersions such as distribution of particle density cannot be observed.
This is due to the fact that the antenna used for the radar described in patent document 1 generally has a diameter of several meters. Furthermore, when performing volume scanning with an antenna having such a large diameter, the scale of a mechanism that drives the antenna is large and the structure of the antenna device is thus complicated. Furthermore, the entire antenna device increases in size and weight. This raises costs.
Additionally, it would be difficult to install many radars including such large and costly antenna devices. Thus, to cope with such a problem, the transmission power can be increased to between several tens of watts and several kilowatts to increase the observation area (observable distance) and thereby enlarge the observation area. However, this would further raise costs.    [Patent Document 1] Japanese Laid-Open Patent Publication No. 11-14749